The present invention relates to the art of manufacturing and formulating photographic films employed in lithographic industry; and more particularly is related to positive film formulations to make color selection by means of balanced primary colors, and to the method of obtaining said formulations.
It is well known in the present state of the art that the basic principle of color printing in the lithographic industry resides in the use of only three primary colors, which results in the illusion of finding in the printed reproduction, the hundreds of colors the human eye is capable of differentiate in the original.
Modern color printing, wherein only three printing inks are used of different colors, was created when the discoveries of photography, half-tone screen, coal tar and synthetic dyes to make transparent color inks, as well as color separation filters were combined to each other. All of these technologies are supported by a better comprehension of the color vision.
All of the commercial color printing processes use a fourth black ink printing, whereby they are considered as processes based on tetrachromatism.
Color printing with inks consists in provide in a controlled manner, red, green and blue light, which is attained in an indirect manner by means of a substractive process.
Printing is carried out with yellow, magenta and cyan dyes, as they are the ideal complementary of blue, green and red colors, respectively; and each is capable of offset the red, green and blue light reflection from the paper surface.
The first step in any color reproduction system is called color separation. This step can be carried out by taking three photographic images of the original, each of them through different red, green or blue filters. Every separation image thus filtered is exposed through a half-tone screen in order to create dots of different sizes to control the tints and values of the different shades.
When these separation images and half-tone screen are simultaneously made on the same photographic film, it is referred to as a direct screen.
In the indirect process, color separation negatives are normal continuous-tone photographic images and the half-tone dots screen is introduced in a second stage, by means of a high-contrast film, in order to create dotted positive images.
It is important to mention that the direct screen method is more economical and, therefore, is the more broadly used.
Nowadays, as already mentioned above, lithographic color printing is made by employing four prints: three of them by using inks corresponding to the three primary colors, and a fourth print by using black ink (tetrachromatism or four-color printing), and to this issue it is necessary that three photographic images are employed of the original to be reproduced by using three different filters, one for every primary color. This means that four photographic films will be required for the lithographic printing, including black color; four processes and the making of four printing sheets, as well as four passes through the printing machine.
In order to make the separation negatives, positive photographic films are presently employed using formulations based on an emulsion sensitive to any type of light, wherein the emulsion used covers the entire range of said three primary colors; whereby each photographic print required for separation of colors has the drawback of not taking advantage of the positive photographic film emulsion in a 100%, as a portion of the emulsion is wasted away, and this impacts directly on the total process cost.
As a consequence from the above, for long it has been sought to overcome the drawbacks of the color printing methods of the state of the art by providing positive film formulations for obtaining color selection by employing balanced primary colors, as well as the method to obtain said formulations; by means of which the printing in a lithographic workshop can become more efficient and economical upon a reduction in the process stages required for the printing, and also upon a reduction in the raw materials required in the manufacture of the positive films employed during the color lithographic printing.
Having in mind the drawbacks of the prior art, it is an object of this invention to provide positive film formulations for obtaining color selection, by employing balanced primary colors, which facilitate a more efficient color printing in the lithographic industry, upon a reduction in the process stages required to carry out said printing.
It is another object of this invention to provide positive film formulations for obtaining color selection by employing balanced primary colors; by means of which more economical color printings can be made in the lithographic industry, due to the reduction in the number of raw materials required in the manufacture of those positive photographic films employed during the color printing.
It is another further object of this invention to provide a method of obtaining positive film formulations to make the color selection by employing balanced primary colors, by means of which a film formulation can be obtained for the gray equivalent which blocks the same amount of light for each of the primary colors.
It is also another object of this invention to provide a method of obtaining positive film formulations to make color selection, by employing balanced primary colors, by means of which a film formulation can be obtained for the gray equivalent which blocks the same amount of light for a mixture of primary colors.
The above as well as other further objects and advantages of the invention are accomplished by means of the positive film formulations for obtaining color selection, through the use of balanced primary colors; with said formulations comprising an emulsion sensitive to any type of light and a photographically acceptable vehicle; said formulations being obtained through a method basically comprising the obtaining of the gray equivalent for every primary color or a mixture of primary colors; there being obtained said equivalent grays by the application of a color volume calculated as per the following mathematical expression:                     V        =                              T            2                    10                                    (        I        )            
wherein: